Add real-time CPU time allocation support

[agners]

With home-assistant/supervisor#2602 and #1236 CPU real-time allocation support got added using cgroupsv1 and CONFIG_RT_GROUP_SCHED.

Unfortunately, cgroupsv2 did not offer a replacement, so it got removed with #1329.

We should check if this is still true in 2025.

[agners]

The #1235 (comment) explains how allocation exactly worked with cgroupsv1.

[agners]

Hm, it seems CGroups v2 still does not have the necessary batteries included:

WARNING: cgroup2 cpu controller doesn’t yet support the (bandwidth) control of realtime processes.

From: https://docs.kernel.org/admin-guide/cgroup-v2.html#cpu

There is also a corresponding Docker issue: moby/moby#48259. I think for now we simply can't do this 😢